A common behavior of thermoelectric layered cobaltites: anincommensurate spin density wave state detected by muon spin rotation andrelaxation

J. Sugiyama¹, H. Itahara¹, J. H. Brewer²,E. J. Ansaldo³, K. Dohmae¹, C. Xia¹, Y. Seno¹,

B. Hitti⁴, T. Tani¹

¹Toyota Central Research and Development Labs. Inc.,Nagakute, Aichi 480-1192, Japan

²TRIUMF, CIAR and Department of Physics and Astronomy,University of British Columbia, Vancouver, BC, V6T 1Z1 Canada

³Department of Physics, University of Saskatchewan,Saskatoon, SK, S7N 5A5 Canada

⁴TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T 2A3 Canada

Magnetism of misfit layeredcobaltites, [Ca₂CoO₃]_0.62^RS[CoO₂]and [Ca₂Co_4/3Cu_2/3O₄]_0.62^RS[CoO₂](RS means a rocksalt-type block), was investigated by a positive muon spinrotation and relaxation (m⁺SR) experiment. For [Ca₂CoO₃]_0.62^RS[CoO₂],a transition to an incommensurate (IC) spin density wave (SDW) state was foundbelow 100 K (= T_SDW^on)and a clear oscillation due to a static internal magnetic field was observedbelow 30 K (= T_SDW), while T_SDW^on = 180 K and T_SDW = 140 K for [Ca₂Co_4/3Cu_2/3O₄]_0.62^RS[CoO₂].The large transition width (= 40 - 70 K) suggested that a short-range IC-SDWorder appeared below T_SDW^on anda long-range order completed below T_SDW. Furthermore, an anisotropic behavior of the zero-field m⁺SR spectra on the c axis aligned samples indicated that the IC-SDWpropagates in the a-b plane, with oscillatingmoments directed along the c axis. Theseresults suggest that the IC-SDW exist not in the rocksalt-type block but in theCoO₂ plane.